Allow freenect_set_tilt_degs to take a negative angle

I can't set a negative angle using gcc 4.8.2 for ARM since negative double values get clamped to 0x0 when casted to uint16_t, as one would expect. Oddly it works on a x86_64 build.

Fix the issue by casting the double to int16_t instead.

Signed-off-by: Jocelyn Turcotte <turcotte.j@gmail.com>