iwlwifi: acpi: move code that reads SPLC to acpi

Move most of the set_dflt_pwr_limit() function to acpi.c and make it
return the pwr_limit value instead of setting directly.  Also rename
it to iwl_acpi_get_pwr_limit().

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>