staging: sm750fb: update license

The driver was actually released with BSD license. It also gained GPL
when it was submitted to be included in the kernel.

Cc: Teddy Wang <teddy.wang@siliconmotion.com>
Cc: gzhou1 <guojian.zhou@windriver.com>
Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>