drivers: wifi: eswifi: Fix build issue due to -fno-common
drivers: wifi: eswifi: Fix build issue due to -fno-common

The introduction of -fno-common caused a link error in eswifi driver due
to lack of missing extern on eswifi_bus_ops_spi.  Adding the extern
resolves the link error.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

finikorg/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,galak/zephyr,nashif/zephyr,punitvara/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,punitvara/zephyr,explora26/zephyr,explora26/zephyr,ldts/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,finikorg/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,ldts/zephyr,punitvara/zephyr,galak/zephyr,explora26/zephyr,nashif/zephyr,Vudentz/zephyr,explora26/zephyr,ldts/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr