msm: footswitch-pcom: Set rail modes only for probed footswitches

Currently, all footswitches supported by footswitch-pcom are enabled
and set to "manual" mode during a subsys_initcall, regardless of
whether or not the footswitch's device will ever probe or even exist
in hardware.

Because calls to enable footswitches also implicitly enable clocks (on
the modem side), this behaviour results in clocks being turned on
unnecessarily. The clock driver's "auto-off" feature later disables
them, but it's best not to rely on that happening (especially since
auto-off may soon be removed).

Change-Id: Ie3f59ddcba44a126cf74dd64e5e50848ca61e364
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>