wifi: mt76: mt792x: fix NULL dereference in ACPI SAR init during probe

wifi: mt76: mt792x: fix NULL dereference in ACPI SAR init during probe

Some laptops carry a MediaTek power table in their firmware, and the
driver reads it to set a transmit limit for each frequency range.  It
only fills in the ranges themselves when it registers the device.

The startup step that does this existed already, but it never programmed
anything.  Two recent commits made it run a regulatory update instead,
which sets the limits on the way through, long before registration.

As a result, on a machine that has the table the driver reads through an
empty pointer and the interface never appears:

  BUG: kernel NULL pointer dereference, address: 0000000000000004
  RIP: 0010:mt792x_init_acpi_sar_power
  Call Trace:
   mt7921_set_tx_sar_pwr
   mt7921_mcu_regd_update
   mt7921_regd_update
   mt7921_run_firmware
   mt7921e_mcu_init
   mt7921_init_work

Skip it when the ranges are missing. They are applied again once the
device is up, which is where they came from before.

Reported-by: Klara Modin <klarasmodin@gmail.com>
Closes: https://lore.kernel.org/linux-wireless/aoyxqHYvSuaBeubf@soda.int.kasm.eu/
Fixes: 9b80bd9cab40 ("wifi: mt76: mt7921: add regulatory wiphy self manager support")
Fixes: e9f3f1cc133f ("wifi: mt76: mt7925: add regulatory wiphy self manager support")
Signed-off-by: Devin Wittmayer <lucid_duck@justthetip.ca>
Tested-by: David Gow <david@davidgow.net>
Tested-by: Klara Modin <klarasmodin@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>