wifi: mt76: fix broken precal loading from MTD for mt7915

wifi: mt76: fix broken precal loading from MTD for mt7915

Commit 495184ac91bb ("mt76: mt7915: add support for applying
pre-calibration data") was fundamentally broken and never worked.

The idea (before NVMEM support) was to expand the MTD function and pass
an additional offset. For normal EEPROM load the offset would always be
0. For the purpose of precal loading, an offset was passed that was
internally the size of EEPROM, since precal data is right after the
EEPROM.

Problem is that the offset value passed is never handled and is actually
overwrite by

	offset = be32_to_cpup(list);
	ret = mtd_read(mtd, offset, len, &retlen, eep);

resulting in the passed offset value always ingnored. (and even passing
garbage data as precal as the start of the EEPROM is getting read)

Fix this by adding to the current offset value, the offset from DT to
correctly read the piece of data at the requested location.

Cc: stable@vger.kernel.org
Fixes: 495184ac91bb ("mt76: mt7915: add support for applying pre-calibration data")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>