sony-laptop: Fix handling sony_nc_hotkeys_decode result

sony_nv_hotkeys_decode can return a negative value. real_ev is a u32 variable.
The check for real_ev > 0 is incorrect.

Use an intermediate ret variable.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/assign_signed_to_unsigned.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2046107



Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
[dvhart: clarify commit msg, drop superfluous else block]
Signed-off-by: Darren Hart <dvhart@linux.intel.com>