rsi: fix memory leak on buf and usb_reg_buf

In the cases where len is too long, the error return path fails to
kfree allocated buffers buf and usb_reg_buf.  The simplest fix is to
perform the sanity check on len before the allocations to avoid having
to do the kfree'ing in the first place.

Detected by CoverityScan, CID#1452258,1452259 ("Resource Leak")

Fixes: 59f73e2a

 ("rsi: check length before USB read/write register")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>