regulator: rpmh-regulator: fix rpmh_regulator_init_vreg_supply memory leak

Commit 29c41d5c ("regulator: rpmh-regulator: support parent
supply specification in subnodes") introduced a memory leak in
the function rpmh_regulator_init_vreg_supply() due to 'buf' not
being freed in the new 'else if' statement.  Fix this memory leak
by freeing 'buf' correctly.

Change-Id: If2748b595d8ae3a4564a2956a67aeda476975309
Signed-off-by: David Collins <collinsd@codeaurora.org>