env: mmc: Change guards around CONFIG_SYS_MMC_ENV_PART

With a move to having CONFIG_SYS_MMC_ENV_PART to Kconfig, it will always
be defined.  While in theory eMMC will almost always have multiple
partitions, in the case where we have not configured the system to have
environment stored on a partition other than 0 (user), we don't need to
worry about switching to that partition from where we may otherwise be.

Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Wolfgang Denk <wd@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>