IB/hfi1: Move a function from a header file into a .c file

Since ib_get_len() only has one caller, move it from a header file into a
.c file. Additionally, remove the superfluous u16 cast. That cast was
introduced by commit 7dafbab3 ("IB/hfi1: Add functions to parse BTH/IB
headers").

Link: https://lore.kernel.org/r/20210524041211.9480-2-bvanassche@acm.org


Cc: Don Hiatt <don.hiatt@intel.com>
Cc: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>