usb: misc: usbtest: remove duplicate & operation

usb_endpoint_maxp() has an inline keyword and searches for bits[10:0]
by & operation with 0x7ff. So, we can remove the duplicate & operation
with 0x7ff.

Signed-off-by: Jaejoong Kim <climbbb.kim@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>