usb: ehci-omap: don't hard-code TLL channel count

Make the TLL channel count a parameter instead of a hardcoded
value. This allows us to be flexible with future OMAP revisions
which could have a different number of channels.

Signed-off-by: Keshava Munegowda <keshava_mgowda@ti.com>
Signed-off-by: Anand Gadiyar <gadiyar@ti.com>