staging: sm750fb: change definition of SYSTEM_CTRL multi-bit fields

Use more straight-forward definitions for multi-bit fields of
SYSTEM_CTRL register and replace FIELD_GET/SET for these fields with
open-coded implementation.

Signed-off-by: Mike Rapoport <mike.rapoport@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>