net: lan78xx: Avoid unnecessary self assignment

Clang warns when a variable is assigned to itself.

drivers/net/usb/lan78xx.c:940:11: warning: explicitly assigning value of
variable of type 'u32' (aka 'unsigned int') to itself [-Wself-assign]
                        offset = offset;
                        ~~~~~~ ^ ~~~~~~
1 warning generated.

Reorder the if statement to acheive the same result and avoid a self
assignment warning.

Link: https://github.com/ClangBuiltLinux/linux/issues/129


Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>