staging: rtl8712: Fix unnecessary parentheses style warning in rtl871x_xmit.c

This fixes the following checkpatch.pl warning:
WARNING: Unnecessary parentheses - maybe == should be = ?

Signed-off-by: Esra Altintas <es.altintas@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>