mux: adg792a: always require I2C support

COMPILE_TEST makes no sense when I2C is disabled, as the driver cannot compile
in that configuration:

drivers/mux/mux-adg792a.c: In function 'adg792a_write_cmd':
drivers/mux/mux-adg792a.c:34:9: error: implicit declaration of function 'i2c_smbus_write_byte_data'; did you mean 'i2c_set_clientdata'? [-Werror=implicit-function-declaration]
drivers/mux/mux-adg792a.o: In function `adg792a_driver_init':
mux-adg792a.c:(.init.text+0x14): undefined reference to `i2c_register_driver'
drivers/mux/mux-adg792a.o: In function `adg792a_probe':
mux-adg792a.c:(.text.adg792a_probe+0x94): undefined reference to `i2c_smbus_write_byte_data'
drivers/mux/mux-adg792a.o: In function `adg792a_set':
mux-adg792a.c:(.text.adg792a_set+0x80): undefined reference to `i2c_smbus_write_byte_data'

Fixes: afda08c4 ("mux: adg792a: add mux controller driver for ADG792A/G")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>