staging: rtl8712: rtl8712_cmd.c: fixed comparison to null

This patch fixes multiple "comparison to NULL"
checkpatch.pl issues:

CHECK: Comparison to NULL could be written "!pcmd"
+               if (pcmd == NULL)

Signed-off-by: Juliana Rodrigues <juliana.orod@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>