env: mmc: Correct partition comparison in mmc_offset_try_partition

The function mmc_offset_try_partition searches the MMC partition for
locating environment data, by comparing the partition names with config
"u-boot,mmc-env-parition". However, it only compares the first word-size
bytes (size of 'const char *'), which may make the function to find
unintended partition.

Correct the function not to partially compare the partition name with
config "u-boot,mmc-env-partition".

Fixes: c9e87ba6 ("env: Save environment at the end of an MMC partition")
Signed-off-by: Hoyeonjiki Kim <jigi.kim@gmail.com>
Reviewed-by: Wolfgang Denk <wd@denx.de>