ALSA: lx_core: use int type to store negative error codes

ALSA: lx_core: use int type to store negative error codes

Change the 'ret' variable from u16 to int to store negative error codes or
zero returned by lx_message_send_atomic().

Storing the negative error codes in unsigned type, doesn't cause an issue
at runtime but it's ugly as pants. Additionally, assigning negative error
codes to unsigned type may trigger a GCC warning when the -Wsign-conversion
flag is enabled.

No effect on runtime.

Fixes: 02bec4904508 ("ALSA: lx6464es - driver for the digigram lx6464es interface")
Signed-off-by: Qianfeng Rong <rongqianfeng@vivo.com>
Link: https://patch.msgid.link/20250828081312.393148-1-rongqianfeng@vivo.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>