soc: fsl: qe: make cpm_muram_free() ignore a negative offset

This allows one to simplify callers since they can store a negative
value as a sentinel to indicate "this was never allocated" (or store
the -ENOMEM from an allocation failure) and then call cpm_muram_free()
unconditionally.

Reviewed-by: Timur Tabi <timur@kernel.org>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Li Yang <leoyang.li@nxp.com>