sysclk-lwla: Limit use of SR_ERR_ARG to user-supplied arguments.

The return code SR_ERR_ARG is intended for reporting unsupported
or inapplicable device configuration settings and is not a hard
error.  In order to indicate failure of internal sanity checks,
use SR_ERR_BUG instead.
sysclk-lwla: Simplify and optimize word extraction.

It turns out that all LWLA protocol responses consist either
of 32-bit units or of 32-bit units combined into 64-bit units.
Thus it makes sense to double the basic unit size for reading
from 16 bit to 32 bit.
We cannot do the same for command messages though, as those
actually do use 16-bit quantities in some places, and 32-bit
arguments are not always aligned to 32-bit boundaries.

(acquisition_state.xfer_buf_in): Change unit type to uint32_t,
and update related macros and code accordingly.
(LWLA_TO_UINT32): New macro to replace LWLA_READ32, operating
directly on 32-bit values instead of pointers to 16-bit units.
Make use of a compiler-recognized idiom for bitwise rotation
to efficiently swap the 16-bit halves of a 32-bit word.
(LWLA_TO_UINT16): New macro to replace LWLA_READ16.
(LWLA_READ64): Remove unused macro.
(LWLA_WORD_[0123]): Slightly simplify 16-bit word extraction.
sysclk-lwla: Add support for external trigger input.

Implement the configuration setting TRIGGER_SOURCE with the
choices CH (logic channels) and TRG (external trigger input).
Also implement the TRIGGER_SLOPE setting for selecting the
edge to trigger on (rising or falling).