staging: rtl8188eu: core: constify local structure

Constify the static struct RTW_CHANNEL_PLAN_MAP_REALTEK_DEFINE.

The only other instance was one of its properties being assigned to a
variable:
Index2G = RTW_CHANNEL_PLAN_MAP_REALTEK_DEFINE.Index2G;

Therefore this is a good candidate for constification.

Running size did not show any difference:
Before/After:
text    data     bss     dec     hex filename
44122     408    2974   47504    b990

Lastly, break up the long line to fix the checkpatch warning and conform
to kernel coding style.

Signed-off-by: Elizabeth Ferdman <gnudevliz@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>