[PATCH] libata: remove unused HSM_ST_UNKNOWN

HSM_ST_UNKNOWN is not used anywhere.  Its value is zero and supposed
to serve sanity check purpose but HSM_ST_IDLE is used for that
purpose.  This unused state causes confusion.  After a port is
initialized but before the first command is executed, the idle hsm
state is UNKNOWN.  However, once a command has completed, the idle hsm
state is IDLE.  This defeats sanity check in ata_pio_task() for the
first command.

This patch removes HSM_ST_UNKNOWN and consequently make HSM_ST_IDLE
the default state.

Signed-off-by: Tejun Heo <htejun@gmail.com>