net: pcs: xpcs: Return EINVAL in the internal methods

In particular the xpcs_soft_reset() and xpcs_do_config() functions
currently return -1 if invalid auto-negotiation mode is specified. That
value might be then passed to the generic kernel subsystems which require
a standard kernel errno value. Even though the erroneous conditions are
very specific (memory corruption or buggy driver implementation) using a
hard-coded -1 literal doesn't seem correct anyway especially when it comes
to passing it higher to the network subsystem or printing to the system
log.  Convert the hard-coded error values to -EINVAL then.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>