ntb_hw_switchtec: Make function switchtec_ntb_remove() static

Fixes the following sparse warnings:

drivers/ntb/hw/mscc/ntb_hw_switchtec.c:1552:6: warning:
 symbol 'switchtec_ntb_remove' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>