gpio-mcp23s08: Build I2C support even when CONFIG_I2C=m

The driver has both SPI and I2C pieces. The appropriate pieces are built based
on whether SPI and/or I2C is/are enabled. However, it was only checking if I2C
was built-in, never if it was built as a module. This patch checks for either
since building both this driver and I2C as modules is possible.

Signed-off-by: Daniel M. Weeks <dan@danweeks.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>