staging: rtl8723bs: fix warning comparison to NULL

This patch fixes below issue reported by checkpatch

CHECK: Comparison to NULL could be written "!pxmitbuf->pallocated_buf"
+               if (pxmitbuf->pallocated_buf == NULL)

Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>