rtlwifi: Create common routine to get hardware info

All of the rtlwifi family of drivers have a similar routine that acquires
the hardware info from efuse and initializes a number of variables in the
driver's private area. A common routine is created for all drivers to use.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>