Staging: xgifb: XGI_main_26.c: Refactored the function

Refactored the function `XGIfb_search_refresh_rate` by removing a level
of `if...else` block nesting. Removed unnecessary parantheses. Removed
potential bug of array underflow.

Signed-off-by: Pratik Jain <pratik.jain0509@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>