dfu: dfu_sf: Pass duplicate devstr to parse_dev

parse_dev() alters the string pointed by devstr parameter. Due to this
subsequent parsing of sf entities will fail, as string pointed by devstr
is no longer valid sf dev arguments.
Fix this by passing pointer to the copy of the string to parse_dev
instead of pointer to the actual devstr.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>