scsi: ufs: ufshcd: fix potential NULL pointer dereference in ufshcd_config_vreg

_vreg_ is being dereferenced before it is null checked, hence there is a
potential null pointer dereference.

Fix this by moving the pointer dereference after _vreg_ has been null
checked.

This issue was detected with the help of Coccinelle.

Fixes: aa497613 ("ufs: Add regulator enable support")
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Reviewed-by: Subhash Jadavani <subhashj@codeaurora.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>