drm/tiny: sharp-memory: fix pointer error dereference

drm/tiny: sharp-memory: fix pointer error dereference

The function devm_drm_dev_alloc() returns a pointer error upon failure
not NULL. Change null check to pointer error check.

Detected by Smatch:
drivers/gpu/drm/tiny/sharp-memory.c:549 sharp_memory_probe() error:
'smd' dereferencing possible ERR_PTR()

Fixes: b8f9f21716fec ("drm/tiny: Add driver for Sharp Memory LCD")
Signed-off-by: Ethan Tidmore <ethantidmore06@gmail.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patch.msgid.link/20260216040438.43702-1-ethantidmore06@gmail.com