OMAP34xx USB: fix compile breakage in TLL mode

drivers/usb/host/ehci-omap.c in the git tree doesn't compile when
CONFIG_OMAP_EHCI_PHY_MODE is disabled; error message is enclosed below.
Fix the bug, and while there, rearrange the source code to reduce code
duplication and function size.

Also some minor modifications as per David Brownell's comments.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Acked-by: Vikram Pandita <vikram.pandita@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>