staging: wfx: uniformize naming rules in hif_tx_mib.c

staging: wfx: uniformize naming rules in hif_tx_mib.c

hif_tx_mib.c contains functions that format data to be sent to the
hardware. In this file, sometime the struct to be sent is named 'arg',
sometime 'val'. In some other function 'val' is used for the argument of
the function.

This patch uniformize the things and choose to call all the data in
destination to the hardware 'arg' (note this choice is only dictated by
the number of lines to change in the code)

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200907101521.66082-30-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>