tps65090-charger: Fix AC detect

The VACG interrupt was not being enabled.  Thus, interrupts were never
generated when AC status changes.  In addition, interrupts were never
cleared after taking and processing the interrupt.

Added the register offset for the INTR_MASK register, since this is needed
to unmask the VACG interrupt.

Enabled the VACG interrupt in tps65090_config_charger().

Cleared interrupts after processing, in tps65090_charger_isr().

Also removed unused variable "enable" in tps65090_enable_charging(), and
fixed a typo in one of the dev_err() prints.

Signed-off-by: Andrew Chew <achew@nvidia.com>
Tested-by: Rhyland Klein <rklein@nvidia.com>
Acked-by: Rhyland Klein <rklein@nvidia.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>