xilinx: versal-net: fix no previous prototype for function warning.

xilinx: versal-net: fix no previous prototype for function warning.

Included the SPI header to resolve the no previous prototypes
for function. Removed unused mode variable.
sparse warnings
warning: no previous prototype for 'spi_get_env_dev'
[-Wmissing-prototypes]
warning: variable 'mode' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Prasad Kummari <prasad.kummari@amd.com>
Link: https://lore.kernel.org/r/20240905115758.999936-1-prasad.kummari@amd.com
Signed-off-by: Michal Simek <michal.simek@amd.com>