ARM: OMAP2+: powerdomain: avoid testing whether an unsigned char is less than 0

_pwrdm_save_clkdm_state_and_activate() tried to test one of its
unsigned arguments to determine whether it was less than zero.  Fix by
moving the error test to the caller.

Reported-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>