input: atmel_mxt_ts: Don't set mode on unsettable regulators

Currently during boot, and every suspend/resume, we call
regulator_set_optimum_mode on every touchsdcreen regulator. However,
some regulators don't support this function, and this will cause
set_optimum_mode to print an error when this occures.  Correct this by
checking if it is valid to call set_optimum_mode on a given regulator
before doing so.

Change-Id: I8dbd9d8281cb144b31c59d86fcf4e8a8133cd6a7
Signed-off-by: Amy Maloche <amaloche@codeaurora.org>