etnaviv: Avoid infinite loop in find_frame()

etnaviv: Avoid infinite loop in find_frame()

Use of unsigned loop control variable with '>= 0' would lead
to infinite loop.

Reported by clang:

etnaviv_compiler.c:1024:39: warning: comparison of unsigned expression
>= 0 is always true [-Wtautological-compare]
   for (unsigned sp = c->frame_sp; sp >= 0; sp--)
                                   ~~ ^  ~

v2: Simply use the same datatype as c->frame_sp is using.

CC: <mesa-stable@lists.freedesktop.org>
Reported-by: Rhys Kidd <rhyskidd@gmail.com>
Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Rhys Kidd <rhyskidd@gmail.com>