staging: rtl8192u: fix a macro expansion bug

Clang detected this macro expansion bug:
drivers/staging/rtl8192u/r8192U_core.c:2384:76: warning: operator '?:'
	has lower precedence than '+'; '+' will be evaluated first
	[-Wparentheses]

The line from the .c file looks like this:
	u1bAIFS = qos_parameters->aifs[i] * ((mode&(IEEE_G|IEEE_N_24G)) ?9:20) + aSifsTime;

We need to put parenthesis around the entire macro to fix the bug.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>