serial: 8250_exar: Move Exar pieces to custom ->startup()

serial: 8250_exar: Move Exar pieces to custom ->startup()

There is a one more step to consolidate Exar bits under 8250_exar umbrella.
This time we introduce a custom ->startup() callback where the Exar specific
settings are applied.

Cc: Robert Middleton <robert.middleton@rm5248.com>
Cc: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Cc: Aaron Sierra <asierra@xes-inc.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20191011115610.81507-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>