mmc: size optimization when !CONFIG_MMC_SPI

mmc: size optimization when !CONFIG_MMC_SPI

When CONFIG_MMC_SPI is not enabled, the MMC_MODE_SPI capability can
never be set. However there is code in mmc.c which uses the
mmc_host_is_spi macro to check that capability & act accordingly. If we
expand that macro to 0 when CONFIG_MMC_SPI is not set (since it will
always be 0 at runtime anyway) then the compiler can optimize away the
SPI-specific code paths in mmc.c.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>