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

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

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