base-files: find_mmc_part: Silence error when no MMC exists

base-files: find_mmc_part: Silence error when no MMC exists

When running find_mmc_part on a system without MMC or when the given
root device does not exist, an error message is printed to stderr.

    grep: /sys/block/mmcblk*/mmcblk*p*/uevent: No such file or directory

Silence this error message.

Since find_mmc_part is mostly used to get block devices of MMC
partitions and the negative result (partion not found) is checked for by
the caller, the error message can be silenced without causing any
negative impact.

Signed-off-by: Andreas Gnau <andreas.gnau@iopsys.eu>
Link: https://github.com/openwrt/openwrt/pull/19492
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>