agent: Avoid blanks in the ssh key's comment.

* agent/command-ssh.c (card_key_available): Replace blanks.

For managing the authorized_key file of ssh it is convenient if the
comment does not have any spaces. Thus we now return

cardno:FFFE_50FF3D01

instead of

cardno:FFFE 50FF3D01

Note that gpg --export-ssh-key uses the keyid as comment because it
does not known the S/N of the card. Gpg-agent however does not know
about OpenPGP and uses the s/n.