mkefidisk.sh: fix hanging on non-writeable device

mkefidisk.sh: fix hanging on non-writeable device

If cleanup() is called early on, as happens when the device isn't
writeable, then none of the mount point variables are set; thus the
script was calling grep with only one argument and appeared to hang
since it was waiting for input on stdin.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>