serial: sc16is7xx: Put IOControl register into regmap_volatile

serial: sc16is7xx: Put IOControl register into regmap_volatile

According to the IOControl register bits description in the page 31 of
the product datasheet, we know the bit 3 of IOControl register is
softreset, this bit will self-clearing once the reset finish.

In the probe, the softreset bit is set, and when we read this register
from debugfs/regmap interface, we found the softreset bit is still
setting, this confused us for a while. Finally we found this register
is cached, to read the real value from register, we could put it
into the regmap_volatile().

Signed-off-by: Hui Wang <hui.wang@canonical.com>
Link: https://lore.kernel.org/r/20230724034727.17335-1-hui.wang@canonical.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>