tty: n_gsm: Don't ignore write return value in gsmld_output()

tty: n_gsm: Don't ignore write return value in gsmld_output()

[ Upstream commit 9136c68346d096697935b9840782f7051d5796c5 ]

We currently have gsmld_output() ignore the return value from device
write. This means we will lose packets if device write returns 0 or
an error.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20210930060624.46523-1-tony@atomide.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>