staging: r8188eu: prevent array underflow in rtw_hal_update_ra_mask()

The problem is that "mac_id" is a u32 so this check for underflow does
not work when "mac_id" is zero.  In that situation, "mac_id - 1" is
UINT_MAX instead of -1 so the condition is true.  It leads to an
array underflow on the next line.

Fixes: 8cd574e6

 ("staging: r8188eu: introduce new hal dir for RTL8188eu driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20210930122604.GB10068@kili


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>