misc: st_core: Protect unsigned value against becoming negative

Coverity reported:
  This less-than-zero comparison of an unsigned value is never true.

In answer to that, we only ever decrement if protos_registered is
positive.  We can subsequently remove the paranoid checking during
unregister.

Cc: gregkh@linuxfoundation.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>