staging: rtl8188eu: remove get_right_chnl_for_iqk()

The function get_right_chnl_for_iqk() only returns non zero values for
channels > 14. According to the TODO, code valid only for 5 GHz 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 get_right_chnl_for_iqk() and the variable chn_index that is
used to save the return value. Replace the uses of chn_index with zero.

Remove the now unused define ODM_TARGET_CHNL_NUM_2G_5G.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>