usb: dwc3: convert TRBs into bitshifts

this will get rid of a useless memcpy on
IRQ handling, thus improving driver performance.

Tested with OMAP5430 running g_mass_storage on
SuperSpeed and HighSpeed.

Note that we are removing the little endian access
of the TRB and all accesses will be in System endianness,
if there happens to be a system in BE, bit 12 of GSBUSCFG0
should be set so that HW does byte invariant BE accesses
when fetching TRBs.

Signed-off-by: Felipe Balbi <balbi@ti.com>