orinoco: make firmware download less verbose

The firmware download code has been in a couple of releases, without any
significant issues reported in this code.

Convert to use pr_debug, so the messages can be recoverred by defining
DEBUG.

Signed-off-by: David Kilroy <kilroyd@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>