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

Use more straight-forward definitions for multi-bit fields of
CURRENT_GATE register and use open-coded implementation for register
manipulation.

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