drivers: spi: kconfig: Turn pointless 'menuconfig' into 'config'

Same deal as in commit 677f1e6d ("config: Turn pointless/confusing
'menuconfig's into 'config's"), for a newly-introduced SPI_MCUX_FLEXCOMM
symbol.

Also clean up the header to be consistent with recent cleanups.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>