wifi: rtw89: enlarge supported length of read_reg debugfs entry

The register ranges of upcoming chips are different from current, and even
existing chips have different ranges, so support longer length to dump
registers. Then, user space can decide the ranges according to chip.

Since arbitrary length (e.g. 7) would be a little complicated, so simply
make length a multiple of 16. The output looks like

18620000h : 8580801f 82828282 82828282 080800fd

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230519031500.21087-6-pkshih@realtek.com