rtlwifi: Improve debugging by using debugfs

Use debugfs to dump register and btcoex status, and also write registers
and h2c.

We create topdir in /sys/kernel/debug/rtlwifi/, and use the MAC address
as subdirectory with several entries to dump mac_reg, bb_reg, rf_reg etc.
An example is
    /sys/kernel/debug/rtlwifi/00-11-22-33-44-55-66/mac_0

This change permits examination of device registers in a dynamic manner,
a feature not available with the current debug mechanism.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>