IB/core: Add helper to convert port attributes to data rate

IB/core: Add helper to convert port attributes to data rate

Introduce ib_port_attr_to_rate() to compute the data rate in 100 Mbps
units (deci-Gb/sec) from a port's active_speed and active_width
attributes. This generic helper removes duplicated speed-to-rate
calculations, which are used by sysfs and the upcoming new verb.

Signed-off-by: Or Har-Toov <ohartoov@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Edward Srouji <edwards@nvidia.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>