staging: sm750fb: Refine code in set_current_gate().

The 'switch' statement in set_current_gate() had only two possible
scenarios, so it was replaced with an 'if' statement to make the code
shorter and easier to understand.

Signed-off-by: Elise Lennion <elise.lennion@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>