ssb: Fix range check in sprom write

The range check in the sprom image parser hex2sprom() is broken.
One sprom word is 4 hex characters.
This fixes the check and also adds much better sanity checks to the code.
We better make sure the image is OK by doing some sanity checks to avoid
bricking the device by accident.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>