p54pci: fix -Wunused-but-set-variable warnings

p54pci.c: In function ‘p54p_tx’:
p54pci.c:334:6: warning: variable ‘device_idx’ set but not used

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>