MMC: make b_max unconditional

Make existing field b_max field in struct mmc unconditional
and use it instead of CONFIG_SYS_MMC_MAX_BLK_COUNT in mmc_bread
and mmc_bwrite.

Initialize b_max to CONFIG_SYS_MMC_MAX_BLK_COUNT in mmc_register
if it has not been initialized by the hw driver.

Initialize b_max to 0 in all callers to mmc_register.

Signed-off-by: John Rigby <john.rigby@linaro.org>
Signed-off-by: Andy Fleming <afleming@freescale.com>