net: sfp: initialize sfp->i2c_block_size at sfp allocation
sfp->i2c_block_size is initialized at SFP module insertion in
sfp_sm_mod_probe(). Because of that, if SFP module was never inserted
since boot, sfp_read() call will lead to zero-length I2C read attempt,
and not all I2C controllers are happy with zero-length reads.
One way to issue sfp_read() on empty SFP cage is to execute ethtool -m.
If SFP module was never plugged since boot, there will be a zero-length
I2C read attempt.
# ethtool -m xge0
i2c i2c-3: adapter quirk: no zero length (addr 0x0050, size 0, read)
Cannot get Module EEPROM data: Operation not supported
If SFP module was plugged then removed at least once,
sfp->i2c_block_size will be initialized and ethtool -m will fail with
different exit code and without I2C error