usb: u_ether: Fix compile errors

commit f1a1823f
usb: gadget: u_ether: convert into module

changes qlen function definition. and this has to be fixed
accordingly in current u_ether driver.

This patch fixes following compile error in u_ether caused by commit.

drivers/usb/gadget/u_ether.c: In function 'rx_fill':
drivers/usb/gadget/u_ether.c:416:3: error: too few arguments to function 'qlen'
   if (++req_cnt > qlen(dev->gadget))
   ^

drivers/usb/gadget/u_ether.c: In function 'eth_start_xmit':
drivers/usb/gadget/u_ether.c:738:24: error: 'qmult' undeclared (first use in this function)
   if (dev->tx_qlen == (qmult/2)) {

which was caused by commits
commit 79467317949e1621240f632acfb7453783bec2e7
USB: gadget: u_ether: Fix data stall issue in RNDIS tethering mode

commit 68b91e8c54f5c091986c5719631893b10eab760a
usb: u_ether: Add workqueue as bottom half handler for rx data path

Change-Id: Ic4e5a1e08cb688e5a606c7c1895f869d8f887b9f
Signed-off-by: Praneeth Bajjuri <praneeth@ti.com>