regulator: cpr-regulator: Improve CPR interrupt handling

disable_irq() is not an efficient call and could cause potential deadlock.
Improve the interrupt handling by removing disable_irq and enable_irq
completely, and instead, checking the state of the interrupt source
(LOOP_EN and BUSY bits) in ISR to bail out immediately if any condition is
not met.

CRs-Fixed: 630182
Change-Id: Ieedadf4f0db4febc41ab85f383af683bdaf34615
Signed-off-by: Tirupathi Reddy <tirupath@codeaurora.org>
Signed-off-by: Ke Liu <keliu@codeaurora.org>