bt_power: Fix BT enable issue

Fix the potential issue in bluetooth power driver by setting the pointer
variable to NULL if IS_ERR condition containing errno when doing
regulator_get in bt_vreg_init.

CRs-Fixed: 2349213
Change-Id: I58139cb7bbef048b91b4112254d95130dc947702
Signed-off-by: Liuliu Zhao <liulzhao@codeaurora.org>