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

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

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