usb: netusb: eem: Use NET_ETH_MAX_FRAME_SIZE as frame size
usb: netusb: eem: Use NET_ETH_MAX_FRAME_SIZE as frame size

Use standard NET_ETH_MAX_FRAME_SIZE for frame size calculation. This
changes buffer from 1522 to 1514 + 2 + 4. Remove also magic numbers.

Signed-off-by: Andrei Emeltchenko <a6565233ddc88e4fb9c66c1d70743223493f2ed4@intel.com>

GiulianoFranchetto/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,ldts/zephyr,nashif/zephyr,galak/zephyr,ldts/zephyr,Vudentz/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,ldts/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,ldts/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr