Looks like dm81xx can only do 32-bit fifo reads like am35x. Let's set
up musb-dsps with a custom read_fifo function based on the compatible
flag.

Otherwise we can get the following errors when starting dhclient on a
asix USB Ethernet adapter:

asix 2-1:1.0 eth2: asix_rx_fixup() Bad Header Length 0xffff003c, offset 4

While at it, let's also remove pointless cast of the driver data.

Cc: Bin Liu <binmlist@gmail.com>
Cc: Brian Hutchinson <b.hutchman@gmail.com>
Cc: George Cherian <george.cherian@ti.com>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>