mt76: Fix comparisons with invalid hardware key index

With gcc 4.1.2:

    drivers/net/wireless/mediatek/mt76/mt76x0/tx.c: In function ‘mt76x0_tx’:
    drivers/net/wireless/mediatek/mt76/mt76x0/tx.c:169: warning: comparison is always true due to limited range of data type
    drivers/net/wireless/mediatek/mt76/mt76x2_tx_common.c: In function ‘mt76x2_tx’:
    drivers/net/wireless/mediatek/mt76/mt76x2_tx_common.c:35: warning: comparison is always true due to limited range of data type

While assigning -1 to a u8 works fine, comparing with -1 does not work
as expected.

Fix this by comparing with 0xff, like is already done in some other
places.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>