staging: rtl8192u: replace macro rx_hal_is_cck_rate() in r8192U_core.c

This patch replaces macro rx_hal_is_cck_rate() with
the static inline function rx_hal_is_cck_rate().

This replacement was suggested by Dan Carpenter and
has the following benefits:
- improves code readability
- guarantees type safety
- improves code efficiency by enforcing the evaluation of
  the simple boolean expression (!pdrvinfo->RxHT) to be
  done before the evaluation of the more complex one
  (pdrvinfo->RxRate == DESC90_RATE1M ||
   pdrvinfo->RxRate == DESC90_RATE2M ||
   pdrvinfo->RxRate == DESC90_RATE5_5M ||
   pdrvinfo->RxRate == DESC90_RATE11M)

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>