wifi: mt76: mt7921: fix error code of return in mt7921_acpi_read

wifi: mt76: mt7921: fix error code of return in mt7921_acpi_read

Kernel NULL pointer dereference when ACPI SAR table isn't implemented well.
Fix the error code of return to mark the ACPI SAR table as invalid.

[    5.077128] mt7921e 0000:06:00.0: sar cnt = 0
[    5.077381] BUG: kernel NULL pointer dereference, address:
0000000000000004
[    5.077630] #PF: supervisor read access in kernel mode
[    5.077883] #PF: error_code(0x0000) - not-present page
[    5.078138] PGD 0 P4D 0
[    5.078398] Oops: 0000 [#1] PREEMPT SMP NOPTI
[    5.079202] RIP: 0010:mt7921_init_acpi_sar+0x106/0x220
[mt7921_common]
...
[    5.080786] Call Trace:
[    5.080786]  <TASK>
[    5.080786]  mt7921_register_device+0x37d/0x490 [mt7921_common]
[    5.080786]  mt7921_pci_probe.part.0+0x2ee/0x310 [mt7921e]
[    5.080786]  mt7921_pci_probe+0x52/0x70 [mt7921e]
[    5.080786]  local_pci_probe+0x47/0x90
[    5.080786]  pci_call_probe+0x55/0x190
[    5.080786]  pci_device_probe+0x84/0x120

Fixes: f965333e491e ("mt76: mt7921: introduce ACPI SAR support")
Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>