omap2/3/4: ehci: avoid compiler error with touchbook

the early_param() call in board-omap3touchbook.c expands to:

static const char __setup_str_early_touchbook_revision[]
	__section(.init.rodata) _aligned(1) = tbr;
[...]

and we have a non-const variable being added to the
same section:

static struct ehci_hcd_omap_platform_data ehci_pdata
__section(.init.rodata);

because of that, gcc generates a section type conflict
which can (and actually should) be avoided by marking
const every variable marked with __initconst.

This patch fixes that for the ehci_hdc_omap_platform_data.

Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>