cmd: mmc: Check whether arguments are valid numbers in dev subcommand

cmd: mmc: Check whether arguments are valid numbers in dev subcommand

Currently when any of speed_mode, part, or dev fails to be parse as a
number, no error is reported. In this case __init_mmc_device() is called
with weird arguments, probably zeroes if there's no digit prefixing the
argument, which is especially confusing when the invocation occasionally
succeeds.

Let's check whether arguments are valid numbers without trailing
characters. This is quite helpful for speed_mode: it requires an index
instead of a mode name, one may easily pass in a string, which will be
parsed as zero (MMC_LEGACY), without carefully reading the
documentation, then finds the MMC device is under an unexpected mode.

Signed-off-by: Yao Zi <me@ziyao.cc>
Tested-by: Anshul Dalal <anshuld@ti.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>