modules: fix grep error on missing $device/uevent

Issue #33: A race at boot time may cause the removal of an USB device by
the kernel between enumerating all devices and individually accessing
sysfs uevent files. As a consequence, grep reports errors on missing
files during boot. Fix this.
Merge pull request #33 from shuber2/missing-uevent

modules: fix grep error on missing $device/uevent


Looks good. THank you.