Staging: sm750fb: provide error path for hw_sm750le_setBLANK()

This provides a default path for the switch statement in
hw_sm750le_setBLANK() so that the compiler will not correctly complain
about undefined values being sent to the hardware.

Instead, properly error out if the blank command is unknown by the
driver.

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