spi: spi-mem: clean up kernel-doc in spi-mem.h

spi: spi-mem: clean up kernel-doc in spi-mem.h

Eliminate all kernel-doc warnings in spi-mem.h:
- add missing struct member descriptions
- don't use "struct" for function descrptions

Warning: include/linux/spi/spi-mem.h:202 struct member 'cmd' not described
 in 'spi_mem_op'
Warning: include/linux/spi/spi-mem.h:202 struct member 'addr' not described
 in 'spi_mem_op'
Warning: include/linux/spi/spi-mem.h:202 struct member 'dummy' not
 described in 'spi_mem_op'
Warning: include/linux/spi/spi-mem.h:202 struct member 'data' not described
 in 'spi_mem_op'

Warning: include/linux/spi/spi-mem.h:286 Incorrect use of kernel-doc
 format: * struct spi_mem_get_drvdata() - get driver private data
 attached to a SPI mem
Warning: include/linux/spi/spi-mem.h:298 Incorrect use of kernel-doc
 format: * struct spi_controller_mem_ops - SPI memory operations
Warning: include/linux/spi/spi-mem.h:362 expecting prototype for struct
 spi_mem_set_drvdata. Prototype was for struct spi_controller_mem_ops instead

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://patch.msgid.link/20260301014743.3133167-1-rdunlap@infradead.org
Signed-off-by: Mark Brown <broonie@kernel.org>