drivers: ata: Mark the function ahci_init_interrupts() as static in ahci.c

This patch marks the function ahci_init_interrupts() as static in ahci.c
because it is not used outside this file.

Thus, it also eliminates the following warning in ahci.c:
drivers/ata/ahci.c:1099:5: warning: no previous prototype for ‘ahci_init_interrupts’ [-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>