USB: gadget: f_ncm: Fix NDP16 datagram validation

USB: gadget: f_ncm: Fix NDP16 datagram validation

commit 2b74b0a04d3e ("USB: gadget: f_ncm: add bounds checks to ncm_unwrap_ntb()")
adds important bounds checking however it unfortunately also introduces  a
bug with respect to section 3.3.1 of the NCM specification.

wDatagramIndex[1] : "Byte index, in little endian, of the second datagram
described by this NDP16. If zero, then this marks the end of the sequence
of datagrams in this NDP16."

wDatagramLength[1]: "Byte length, in little endian, of the second datagram
described by this NDP16. If zero, then this marks the end of the sequence
of datagrams in this NDP16."

wDatagramIndex[1] and wDatagramLength[1] respectively then may be zero but
that does not mean we should throw away the data referenced by
wDatagramIndex[0] and wDatagramLength[0] as is currently the case.

Breaking the loop on (index2 == 0 || dg_len2 == 0) should come at the end
as was previously the case and checks for index2 and dg_len2 should be
removed since zero is valid.

I'm not sure how much testing the above patch received but for me right now
after enumeration ping doesn't work. Reverting the commit restores ping,
scp, etc.

The extra validation associated with wDatagramIndex[0] and
wDatagramLength[0] appears to be valid so, this change removes the incorrect
restriction on wDatagramIndex[1] and wDatagramLength[1] restoring data
processing between host and device.

Fixes: 2b74b0a04d3e ("USB: gadget: f_ncm: add bounds checks to ncm_unwrap_ntb()")
Cc: Ilja Van Sprundel <ivansprundel@ioactive.com>
Cc: Brooke Basile <brookebasile@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Link: https://lore.kernel.org/r/20200920170158.1217068-1-bryan.odonoghue@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>