TMP: hwspinlock/core: use IS_ERR_OR_NULL()

Commit 352390d hwspinlock/core: return ERR_PTRs on failure in _request_ api
added some error checks where the expression (!hwlock || IS_ERR(hwlock) is
used, this can be replaced with the much simpler IS_ERR_OR_NULL(hwlock)
expression.

TODO: Squash into the original commit.

Signed-off-by: Suman Anna <s-anna@ti.com>