rtw88: 8822c: debug: allow debugfs to enable/disable TXGAPK

Use "cat dm_cap" to show all features; where, prefix +/- means feature is
enabled/disabled:

$ cat dm_cap
DM capability 0x00000002
( 1) +TXGAPK

To control dm_cap:
use "echo +1 > dm_cap" to enable TXGAPK
use "echo -1 > dm_cap" to disable TXGAPK

Below is an example to disable TXGAPK.

$ echo -1 > dm_cap
$ cat dm_cap
DM capability 0x00000000
( 1) -TXGAPK

Below is an example to show TXGAPK status
$ echo 1 > dm_cap; cat dm_cap

( 1) +TXGAPK

path 0:
0x56 = 0x88c89
[TXGAPK] offset 1 0
[TXGAPK] offset 1 1
[TXGAPK] offset 1 2
[TXGAPK] offset 1 3
[TXGAPK] offset 0 4
[TXGAPK] offset 0 5
[TXGAPK] offset 0 6
[TXGAPK] offset 0 7
[TXGAPK] offset 0 8
[TXGAPK] offset 0 9

path 1:
0x56 = 0x89c89
[TXGAPK] offset 1 0
[TXGAPK] offset 1 1
[TXGAPK] offset 1 2
[TXGAPK] offset 1 3
[TXGAPK] offset 0 4
[TXGAPK] offset 0 5
[TXGAPK] offset 0 6
[TXGAPK] offset 0 7
[TXGAPK] offset 0 8
[TXGAPK] offset 0 9

Signed-off-by: Guo-Feng Fan <vincent_fann@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210419003748.3224-4-pkshih@realtek.com