i2ctransfer: Prevent msgs[] overflow with many parameters

There's an off-by-one bug in the message count check to ensure that we
do not process more messages than the kernel allows. nmsgs points to
the index within msgs[] which would be used for the _next_ message. If
this index is equal the maximum number of messages then we must stop
already.

This closes bug #220112:
https://bugzilla.kernel.org/show_bug.cgi?id=220112

Fixes: 9fc53a7fc669 ("i2c-tools: add new tool 'i2ctransfer'")
Signed-off-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>