prism54: prism54_get_encode() test below 0 on unsigned index

previously in this function:

u32 index = (dwrq->flags & IW_ENCODE_INDEX) - 1;

index is unsigned, so if -1, the original test (below) didn't work.

Signed-off-by: Roel Kluin <12o3l@tiscali.nl>
Signed-off-by: John W. Linville <linville@tuxdriver.com>