smc91xx_eeprom: Correct chip detection check.

The smc911x_detect function in /net/driver/net/smc911x.c
returns a 0 if everything was ok (a chip was found) and -1 else.
In the standalone example 'smc911x_eeprom' the return value
of smc911x_detect is interpreted in a different way
(0 for error, !0 as OK).
This leads to the error that the chip will not be detected.

Signed-off-by: Juergen Kilb <j.kilb@phytec.de>
Acked-by: Mike Frysinger <vapier@gentoo.org>

Signed-off-by: Ben Warren <biggerbadderben@gmail.com>