ehci-mxc: Clean up

Clean up ehci-mxc:
 - Remove useless #if's.
 - Fix identation.
 - Issue a #error if used with an unsupported platform.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>