spi: spi-mem: Add missing kdoc argument

spi: spi-mem: Add missing kdoc argument

The "*mem" pointer has been added without description, describe it in
the kdoc comment in order to fix the following W=1 warning:

Warning: drivers/spi/spi-mem.c:594 function parameter 'mem' not
described in 'spi_mem_calc_op_duration'

Fixes: a11a51896572 ("spi: spi-mem: Take into account the actual maximum frequency")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202507310409.fnuQ21qb-lkp@intel.com/
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://patch.msgid.link/20250804095013.409700-1-miquel.raynal@bootlin.com
Signed-off-by: Mark Brown <broonie@kernel.org>