fix build problems with CONFIG_MCP2210_DEBUG_VERBOSE

Builds without CONFIG_MCP2210_DEBUG_VERBOSE were failing. Also, if
CONFIG_MCP2210_DEBUG was enabled but CONFIG_MCP2210_DEBUG_VERBOSE was
not then we were getting an undefined symbol.
Correct problem with transfer_one_message()

When we got a message and rejected it, the SPI subsystem wasn't taking
care of cleanup because it was still expecting that we finalize the
message, even if we reject it.
Minor cleanup & trade #ifdefs for if (IS_ENABLED())