ACPI / TPM: replace open-coded _DSM code with helper functions

Use helper functions to simplify _DSM related code in TPM driver.

This patch also help to get rid of following warning messages:
[  163.509575] ACPI Error: Incorrect return type [Buffer] requested [Package]
(20130517/nsxfeval-135)

But there is still an warning left.
[  181.637366] ACPI Warning: \_SB_.IIO0.LPC0.TPM_._DSM: Argument #4 type
mismatch - Found [Buffer], ACPI requires [Package] (20130517/nsarguments-95)

Signed-off-by: Jiang Liu <[email protected]>
Signed-off-by: Rafael J. Wysocki <[email protected]>

Author: Jiang Liu