mfd: wcd9xxx: avoid slimbus read/write during ADSP reset

During ADSP subsystem restart, all slimbus read/write would fail
as the bus is not available. Avoid codec read/write, until the
slimbus master in ADSP is up.

Change-Id: Iec1933d2625c1cd3dd700b2b3e2ffc7eae64880b
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>