p54pci: fix serious sparse warning

This patch fixes a bug which was just recently introduced by
("p54pci: prevent stuck rx-ring on slow system").

make M=drivers/net/wireless/p54 C=2 CF=-D__CHECK_ENDIAN__
  CHECK   drivers/net/wireless/p54/p54pci.c
drivers/net/wireless/p54/p54pci.c:143:11: warning: cast to restricted __le32
  CC [M]  drivers/net/wireless/p54/p54pci.o

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>