ahci: delete write-only variables (v2)

ahci: delete write-only variables (v2)

Avoid these warnings with GCC 4.6.0:
/src/qemu/hw/ide/ahci.c: In function 'ahci_reset_port':
/src/qemu/hw/ide/ahci.c:810:14: error: variable 'tfd' set but not used [-Werror=unused-but-set-variable]
/src/qemu/hw/ide/ahci.c: In function 'handle_cmd':
/src/qemu/hw/ide/ahci.c:1103:19: error: variable 'pr' set but not used [-Werror=unused-but-set-variable]

In the tfd variable case, fix the logic also.

CC: Alexander Graf <agraf@suse.de>
CC: Kevin Wolf <kwolf@redhat.com>
Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>