mt76: Fix a signedness bug in mt7615_add_interface()

The problem is that "mvif->omac_idx" is a u8 so it can't be negative
and the error handling won't work.  The get_omac_idx() function returns
-1 on error.

Fixes: 04b8e659 ("mt76: add mac80211 driver for MT7615 PCIe-based chipsets")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>