smc911x: update SMC911X related configuration description

Since commit 736fead8fdbf8a8407048bebc373cd551d01ec98 "Convert SMC911X
Ethernet driver to CONFIG_NET_MULTI API" SMC911X configration options
are called CONFIG_SMC911X rather than CONFIG_DRIVER_SMC911X. Update
README to reflect that change.

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
1 file changed