linux_usbfs: Fix parsing of descriptors for multi-configuration devices

linux_usbfs: Fix parsing of descriptors for multi-configuration devices

Commit e2be556bd2 ("linux_usbfs: Parse config descriptors during device
initialization") introduced a regression for devices with multiple
configurations. The logic that verifies the reported length of the
configuration descriptors failed to count the length of the
configuration descriptor itself and would truncate the actual length by
9 bytes, leading to a parsing error for subsequent descriptors.

Closes #825

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>