regulator: qpnp-labibb: Don't handle LAB_VREG_OK in TTW mode for pmi8998

Currently for pmi8998, LAB_VREG_OK interrupt is handled to switch
from PWM to PFM mode after a certain delay to improve efficiency.
Also, PFM mode is kept disabled once the regulator is disabled.
As per the hardware recommendation, keeping LAB in PWM mode is
sufficient in particular when TTW mode is enabled. Hence do not
register for LAB_VREG_OK interrupt in TTW mode so that this
PWM <-> PFM mode transition will not be done.

CRs-Fixed: 2179209
Change-Id: I499740d014a45e02b802d350248f8044a5b6596b
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>