staging: rtl8723bs: remove 5 GHz code

According to the TODO 5 GHz code should be removed.

- find and remove remaining code valid only for 5 GHz. Most of the obvious
  ones have been removed, but things like channel > 14 still exist.

Remove code for channels > 14 from rtw_get_center_ch().

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20200813065053.13883-1-straube.linux@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>