Fixed error when FDISK in check_mmc() returns multiple matches.

When calling mk_mmc.sh with a --device flag where the device name matches the beginning of several devices, the variable FDISK will be assigned several rows of matches:

Ex.
Calling mk_mmc.sh with --device /dev/mmcblk0 on a system where the device tree looks like this:
```
/dev/mmcblk0
/dev/mmcblk0boot1
/dev/mmcblk0boot2
```
will assign FDISK with all those matches. Hence the comparison on line 1273 will fail
even though the device exist.

Example output from the program:
```
./mk_mmc.sh --mmc /dev/mmcblk0 --dtb am335x-boneblack --distro wheezy-armhf --firmware

Are you sure? I Don't see [/dev/mmcblk0], here is what I do see...

fdisk -l:
Disk /dev/mmcblk0: 1920 MB, 1920991232 bytes, 3751936 sectors
Disk /dev/mmcblk0boot1: 1 MB, 1048576 bytes, 2048 sectors
Disk /dev/mmcblk0boot0: 1 MB, 1048576 bytes, 2048 sectors
```
My solution is simple but will clearly fail if the other matches contains anything else than a-z,A-Z,0-9 after the similar characters in their names. Maybe a better regexp would be appropriate.