ARM: OMAP: USB: remove unnecessary sideways include

Remove the include of mach-omap2/common.h from plat-omap/usb.c.  It
appears to be unneeded.  Removing it also resolves the sparse warning:

arch/arm/plat-omap/../mach-omap2/common.h:183:12: warning: symbol 'omap_intc_of_init' was not declared. Should it be static?

Signed-off-by: Paul Walmsley <paul@pwsan.com>