Staging: rtl8192u: ieee80211: Use !x in place of NULL comparisons

Fix the following checkpatch.pl warning :
CHECK: Comparison to NULL could be written "!x"
by changing (x == NULL) to !x and (x != NULL) to x.

Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>