ntb_hw_switchtec: fix the spelling of "its"

Use the possessive "its" instead of the contraction "it's" (it is)
in user messages.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Kurt Schwemmer <kurt.schwemmer@microsemi.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>