env: mmc: Apply GPT only on eMMC user HW partition

Apply the GPT U-Boot environment GUID type look up only on eMMC user
HW partition, do not apply the look up on eMMC boot HW partitions as
mmc_offset_try_partition() assumes either SD partitions or eMMC user
HW partition.

This fixes environment operation on systems where CONFIG_SYS_MMC_ENV_PART
is non-zero and CONFIG_SYS_REDUNDAND_ENVIRONMENT is set.

Fixes: 80105d8f ("env: mmc: select GPT env partition by type guid")
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>