rtw88: add h2c command in debugfs

It's very useful to send H2C command for debug usage. Add an entry
for sending H2C command to firmware.

usage example:
echo 42,00,00,43,02,00,00,00 > /sys/kernel/debug/ieee80211/phyX/rtw88

Signed-off-by: Tzu-En Huang <tehuang@realtek.com>
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200717064937.27966-8-yhchuang@realtek.com