Staging: sm750fb: fix build warning with lynx_accel

Change the return value of lynx_accel to be void, to fix the build
warning, and due to the fact that the function can't seem to fail at
all, and no one cares if it does or not.

Cc: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Cc: Teddy Wang <teddy.wang@siliconmotion.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>