max3110: add sysrq support

This patch moves several occurences of similar code inside receive_chars(),
which now also takes care of checking for break and calling sysrq handling
code.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>