mmc: core: Rename erase_timeout to cmd_timeout_ms.

Renames erase_timeout to cmd_timeout_ms inside struct mmc_command.
First step to making host honor timeouts for non-data-transfer
commands. Cleans up erase timeout code.

Signed-off-by: Andrei Warkentin <andreiw@motorola.com>
Signed-off-by: Chris Ball <cjb@laptop.org>