Change return type of reg_read() and reg_write() callback to ssize_t for
nvmem suppliers to return number of bytes read/written to the nvmem core.
Currently nvmem core assumes the amount of data read/written is equal
to what it has requested from the supplier, this return code facilitates
better error handling in the nvmem core.
Signed-off-by: Joy Chakraborty <joychakr@xxxxxxxxxx>