mmccam: fix mmcsd disk aliases

For EXT_CSD_PART_CONFIG_ACC_BOOT<n> and possibly others with suffixes
we fail to create proper disk aliases (symlinks), which shows up as
g_dev_taste: make_dev_alias_p() failed (name=mmcsd0, error=17)

In this case we ended up with the followng two:

/dev/mmcsd0 -> sdda0
/dev/mmcsd1 -> sdda0boot1

Note that (i) it should be mmcsd0boot1 and not mmcsd1 and that
(ii) there is no mmcsd0boot0 (failed above as it tried to create a
second mmcsd0).

Adjust the code (using a highly simplified version--compared to my
original approach--suggested by imp) using an extended format string
with (sdda/mmcsd) prefix as first argument to create proper names.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D43538

(cherry picked from commit a84d91d81a6f3eeb4949c4fb3440e0634f2b953a)