staging: sm750fb: use BIT macro for VGA_CONFIGURATION single-bit fields

Replace complex defintion of VGA_CONFIGURATION register fields with BIT()
macro and use open-coded implementation for VGA_CONFIGURATION
manipulations.

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