power: smb-lib: fix a mutex deadlock issue

An early error return without releasing a mutex in functon
smblib_vbus_regulator_enable will lead to deadlock in some
situation.

Fix it.

Change-Id: I41178ee3aadfae57566e5f14f81ed82b7b1986cc
Signed-off-by: Harry Yang <harryy@codeaurora.org>