wifi: mt76: mt7915: check return value before accessing free_block_num

Check return value of mt7915_mcu_get_eeprom_free_block() first before
accessing free_block_num.

Fixes: bbc1d415 ("mt76: mt7915: add default calibrated data support")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>