ucom: s/sio/ucom/

Seems like a copy-paste error, or at least this made more sense when the
sio(4) driver still existed. This modifies the debug port name displayed
at boot, but otherwise has no functional change.

Reviewed by: hselasky
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D33278

(cherry picked from commit e43d081f352c97c0cd1c2395f3780601cec09abf)