staging: greybus: spilib: use 'spi_delay_to_ns' for getting xfer delay

staging: greybus: spilib: use 'spi_delay_to_ns' for getting xfer delay

The intent is the removal of the 'delay_usecs' field from the
spi_transfer struct, as there is a 'delay' field that does the same
thing.

The spi_delay_to_ns() can be used to get the transfer delay. It works by
using the 'delay_usecs' field first (if it is non-zero), and finally
uses the 'delay' field.

Since the 'delay_usecs' field is going away, this change makes use of the
spi_delay_to_ns() function. This also means dividing the return value of
the function by 1000, to convert it to microseconds.
To prevent any potential faults when converting to microseconds and since
the result of spi_delay_to_ns() is int, the delay is being computed in 32
bits and then clamped between 0 & U16_MAX.

Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20210308145502.1075689-7-aardelean@deviqon.com
Signed-off-by: Mark Brown <broonie@kernel.org>