[SCSI] Add PPR support to spi_print_msg

Introduce a new helper, print_nego() to handle SDTR/WDTR/PPR.
Split out the guts of show_spi_transport_period_helper() into period_to_str()
and use it in print_nego to get the period factor conversion right.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>