staging: sm750fb: Correctly set CLOCK_PHASE bit of display controller.

Commit 6fba39cf ("staging: sm750fb: use BIT macro for
PANEL_DISPLAY_CTRL single-bit fields") accidentally changed the
CLOCK_PHASE logic from '|=' to '=' which clears all the previously set
bits.

Fixes: 6fba39cf ("staging: sm750fb: use BIT macro for PANEL_DISPLAY_CTRL single-bit fields")
Signed-off-by: Phil Turnbull <phil.turnbull@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>