staging: r8188eu: fix warnings in rtw_wlan_util

Refactor the code to fix following warnings:
    WARNING: Comparisons should place the constant on the right side of the test
    CHECK: Comparison to NULL could be written "p"

Signed-off-by: Vihas Makwana <makvihas@gmail.com>
Link: https://lore.kernel.org/r/20220517140626.3716-4-makvihas@gmail.com


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