Fix recognition of SD/MMC device names

Quoting the commit message:

User reported that GParted didn't detect their eMMC drive [1]. Not
recognised device name was /dev/mmcblk0. Confirmed that the regression
was introduced by this commit [2]. Fix the code and regular expression
used to recognise SD/MMC device names.

[1] GParted forum thread: eMMC drive not detected...?
http://gparted-forum.surf4.info/viewtopic.php?id=17994

[2] 52930f30
Refactor load_proc_partitions_info_cache() a bit (#131 (closed))