sc16is7xx: Handle modem status lines

sc16is7xx: Handle modem status lines

The uart_handle_cts_change() and uart_handle_dcd_change() must be called
with port lock being held. Acquire the lock after reading MSR register.
Do not acquire spin lock when reading MSR register because I2C/SPI port
functions cannot be called with spinlocks held.

Update rng and dsr counters. Wake up delta_msr_wait to allow tty notice
modem status change.

Co-developed-by: Lech Perczak <l.perczak@camlintechnologies.com>
Co-developed-by: Tomasz Moń <tomasz.mon@camlingroup.com>
Signed-off-by: Lech Perczak <l.perczak@camlintechnologies.com>
Signed-off-by: Tomasz Moń <tomasz.mon@camlingroup.com>
Link: https://lore.kernel.org/r/20220301060332.2561851-3-tomasz.mon@camlingroup.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>