i2c/sysfs fd read/write: remove warning

The file descriptor variable is considered as not-initialized in
some functions. As we use the -Werror compilation switch, it produces
this message:
error: 'fd' may be used uninitialized in this function

Correct this by giving the value -ENODEV to initilize fd with a value.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>