Support the Vodafone R215 LET USB dongle, which is apparently a rebadged
E5372 with different product IDs.

Interestingly, the standard E5372 IDs (12d1:1506) are currently listed in
u3g.c and are the same as the E3131.  However, the R215/E5372 is an NCM
device and works well with cdce(4) whereas the E3131 isn't.  More work
may be needed to better identify the other device IDs.

MFC after:	1 week
Remove call to access(2) which didn't serve any purpose, and make it more
tolerant to errors.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation