usb: phy: Fix deadlock because of mutex recursion

The driver acquires the lock on same mutex twice in
the DP DM regulator enable and disable call.
Fix this my removing the mutex_lock and unlock from
dpdm_regulator enable and disable calls.

Change-Id: Ib60c870a5026eac27d872161b22fc3f7f9605fc4
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>