sc16is7xx: fix FIFO address of secondary UART

Calls to regmap_raw_read/write needed register rewrite in a
similar way as function calls to regmap_read/write already had.
This enables reading/writing the serial datastream to the device.

Signed-off-by: Bo Svangård <bo.svangard@embeddedart.se>
Signed-off-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Jon Ringle <jringle@gridpoint.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>