spi: bcm53xx: use msecs_to_jiffies for conversion

Converting milliseconds to jiffies by "val * HZ / 1000" is technically
ok but msecs_to_jiffies(val) is the cleaner solution and handles all
corner cases correctly.

This is only an API consolidation and should make things more readable

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>