drivers: ata: Mark the function as static in libahci.c

This patch marks the function ahci_port_intr() and
ahci_hw_port_interrupt() as static in libahci.c because they are not
used outside this file.

Thus, it also eliminates the following warnings in libahci.c:
drivers/ata/libahci.c:1767:6: warning: no previous prototype for ‘ahci_port_intr’ [-Wmissing-prototypes]
drivers/ata/libahci.c:1800:6: warning: no previous prototype for ‘ahci_hw_port_interrupt’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>