mt76: overwrite default reg_ops if necessary

Introduce mt76_register_debugfs_fops routine in order to
define per-driver regs file operations and make sure the
device is up before reading or writing its registers

Fixes: 1d8efc74 ("mt76: mt7921: introduce Runtime PM support")
Fixes: de5ff3c9 ("mt76: mt7615: introduce pm_power_save delayed work")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>