IB/srp: replace custom implementation of hex2bin()

There is no need to have a duplication of the generic library, i.e.
hex2bin().

Replace the open coded variant.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>