ARM: OMAP2+: gpmc-smsc911x: only register regulator for first instance

commit e4b0b2cb (ARM: OMAP2+: gpmc-smsc911x: add required smsc911x
regulators) added regulators which are registered during
gpmc_smsc911x_init().  However, some platforms (OMAP3/Overo) have more
than one instance of the SMSC911x and result in attempting to register
the same regulator more than once which causes a panic().

Fix this by only registering the regulator when the platform_data id
field is zero, indicating its the first instance.

Cc: Matt Porter <mporter@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>