IB/mthca: Convert to use be16_add_cpu()

replace:

	big_endian_variable = cpu_to_beX(beX_to_cpu(big_endian_variable) +
						expression_in_cpu_byteorder);

with:

	beX_add_cpu(&big_endian_variable, expression_in_cpu_byteorder);

Generated with a semantic patch.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: Sean Hefty <sean.hefty@intel.com>
Cc: Hal Rosenstock <hal.rosenstock@gmail.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>