drivers: ethernet: mcux: fix build warning
drivers: ethernet: mcux: fix build warning

The ethernet HAL has a different uint32_t typedef than Zephyr's u32_t:
uint32_t in the HAL is long unsigned int, while in Zephyr it's
unsigned int. This is causing a build warning on GCC ("warning:
passing argument 2 of ‘ENET_GetRxFrameSize’ from incompatible pointer
type") when passing a u32_t* where ENET_GetRxFrameSize expects a
uint32_t*.

Add a cast to silence the warning.

Signed-off-by: Marti Bolivar <9d90ac4c7bf6a305f6bfd81a23c7859bc883380e@linaro.org>

punitvara/zephyr,aceofall/zephyr-iotos,punitvara/zephyr,runchip/zephyr-cc3220,explora26/zephyr,aceofall/zephyr-iotos,aceofall/zephyr-iotos,aceofall/zephyr-iotos,GiulianoFranchetto/zephyr,mbolivar/zephyr,kraj/zephyr,mbolivar/zephyr,Vudentz/zephyr,runchip/zephyr-cc3220,zephyrproject-rtos/zephyr,explora26/zephyr,rsalveti/zephyr,galak/zephyr,nashif/zephyr,fbsder/zephyr,rsalveti/zephyr,runchip/zephyr-cc3220,zephyriot/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,ldts/zephyr,finikorg/zephyr,Vudentz/zephyr,punitvara/zephyr,punitvara/zephyr,kraj/zephyr,finikorg/zephyr,mbolivar/zephyr,fbsder/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,kraj/zephyr,nashif/zephyr,zephyriot/zephyr,rsalveti/zephyr,nashif/zephyr,rsalveti/zephyr,nashif/zephyr,fbsder/zephyr,aceofall/zephyr-iotos,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,explora26/zephyr,Vudentz/zephyr,explora26/zephyr,ldts/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,runchip/zephyr-cc3220,ldts/zephyr,galak/zephyr,Vudentz/zephyr,rsalveti/zephyr,galak/zephyr,finikorg/zephyr,ldts/zephyr,runchip/zephyr-cc3220,finikorg/zephyr,zephyriot/zephyr,fbsder/zephyr,kraj/zephyr,mbolivar/zephyr,finikorg/zephyr,Vudentz/zephyr,mbolivar/zephyr,fbsder/zephyr,zephyrproject-rtos/zephyr