ata: libahci: make ahci_pmp_retry_softreset() as static

sparse says:

drivers/ata/libahci.c:1390:5: warning:
 symbol 'ahci_pmp_retry_softreset' was not declared. Should it be static?

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>