sfc: Use mutex_lock_interruptible() for ethtool EEPROM access

ethtool must contend with the MTD driver for the SPI bus lock, which
may carry out long operations such as flash erase.  Allow it to be
interrupted while waiting.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>