USB: mos7720, mos7840: remove redundant urb check

Coverity (1709, 1710, 1711, 1712, 1713) actually flagged these as
REVERSE_INULLs (NULL check performed after dereference). But looking at
the other drivers I can't see any similar tests and the USB core already
makes sure urb is non-null - so might as well get rid of the checks.

Signed-off-by: Florin Malita <fmalita@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>