serial: sc16is7xx: split into core and I2C/SPI parts (sc16is7xx_regcfg)

Since each I2C/SPI probe function can modify sc16is7xx_regcfg at the same
time, change structure to be constant and do the required modifications on
a local copy.

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://lore.kernel.org/r/20240409154253.3043822-6-hugo@hugovil.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>