platform/x86: surface3_power: Fix always true condition in mshw0011_space_handler()

smatch warnings:
.../surface3_power.c:417 mshw0011_space_handler() warn: always true condition '(ret >= 0) => +(0-u32max >= 0)'

Refactor error handling returned by mshw0011_adp_psr() to avoid always
true condition.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>