mt7601u: don't warn about devices without per-rate power table

We expect EEPROM per-rate power table to be filled with
s6 values and warn user if values are invalid.  However,
there appear to be devices which don't have this section
of EEPROM initialized.  In such case we should ignore
the values and leave the driver power tables set to zero.

Note that vendor driver doesn't care about this case but
mt76x2 skips 0xff per value.  We take mt76x2's approach.

Signed-off-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>