firmware: arm_scmi: quirk: Improve quirk range parsing

firmware: arm_scmi: quirk: Improve quirk range parsing

When a range contains only an end ("-X"), the number string is parsed
twice, as both "sep == first" and "sep != last" are true.  Fix this by
dropping the superfluous number parsing for "sep == first".

This does have a harmless functional impact for the unbounded range:
"-" is now accepted, while it was rejected before.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/fe257b3b7b7b5c17fd0e5727bb9746c731bd7e3c.1775205358.git.geert+renesas@glider.be
(sudeep.holla: Initialise ret to 0 as it will be uninitialise for "-" range)
Signed-off-by: Sudeep Holla <sudeep.holla@kernel.org>