dmaengine: dmatest: split struct dmatest_info from variable declaration

dmaengine: dmatest: split struct dmatest_info from variable declaration

Combining the struct definition with its variable initializer confuses the
kernel-doc parser because __MUTEX_INITIALIZER() expands to contain braces,
breaking brace counting and causing:

  Warning: drivers/dma/dmatest.c:152 struct member '' not described in 'dmatest_info'

Split into separate struct definition and variable declaration, which is
the standard kernel pattern.

Assisted-by: Opencode:Big-pickle
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Link: https://patch.msgid.link/20260530200322.7584-1-rosenp@gmail.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>