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

Replace complex definition of SYSTEM_CTRL fields and usage of
FIELD_GET/SET with BIT() macro and open-coded register value modifications

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