mt76: mt7915: fix potential overflow of eeprom page index

If total eeprom size is divisible by per-page size, the i in for loop
will exceed max page index, which happens in our newer chipset.

Fixes: 26f18380 ("mt76: mt7915: add support for flash mode")
Signed-off-by: Bo Jiao <bo.jiao@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>