libata debugging: set initial dbg value

This patch sets the prerequisites for the new debugging scheme that more or
less resembles Donald Becker's net driver example. This one liner doesn't
change any functionality beside setting the appropriate debug level for the
msg_enable control in the ata_port struct, which will be later used by the
ata_msg_* macros to control the amount of debug information sent to printk.

Signed-off-by:  <petkov@uni-muenster.de>
Signed-off-by: Jeff Garzik <jeff@garzik.org>