usb: musb: gadget: use variables according to their names in rxstate()

In rxstate(), improper types are given to 'fifo_count' and 'len' variables, and
these variables are not used in accordance to their names (up to the certain
point), i.e. 'len' to hold the size of a packet in the RX FIFO, and 'fifo_count'
to hold a difference between 'request->length' and 'request->actual'...
Interchange the variables up to the point where their use starts to make sense
again.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>