serial: sc16is7xx: compile I2C when REGMAP_I2C is module

drivers/tty/serial/sc16is7xx.c:1060:12: warning: 'sc16is7xx_probe' defined but not used [-Wunused-function]
 static int sc16is7xx_probe(struct device *dev,
            ^
drivers/tty/serial/sc16is7xx.c:1176:12: warning: 'sc16is7xx_remove' defined but not used [-Wunused-function]
 static int sc16is7xx_remove(struct device *dev)
            ^
drivers/tty/serial/sc16is7xx.c:1215:29: warning: 'regcfg' defined but not used [-Wunused-variable]
 static struct regmap_config regcfg = {
                             ^

Fixed these warnings by removing the `#ifdef CONFIG_REGMAP_I2C' around their
calls as this driver selects REGMAP_I2C in Kconfig. This part of driver just
didn't compile at all when REGMAP_I2C configured as module (CONFIG_REGMAP_I2C
is not defined, just CONFIG_REGMAP_I2C_MODULE).

Signed-off-by: Jan Moskyto Matejka <mq@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>