ACPI: Correct \_SB._OSC bit definition for PRM

According to Platform Runtime Mechanism Specification v1.0 [1],
Page 42, \_SB._OSC bit 21 is used to indicate OS support for PRM.

Update the definition of the PRM support bit in the code to match the
specification.

Link: https://uefi.org/sites/default/files/resources/Platform%20Runtime%20Mechanism%20-%20with%20legal%20notice.pdf

 # [1]
Fixes: 60faa8f1 ("ACPI: Add \_SB._OSC bit for PRM")
Signed-off-by: Aubrey Li <aubrey.li@linux.intel.com>
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>