Input: atmel_mxt_ts - check mem_size before calculating config memory size

Input: atmel_mxt_ts - check mem_size before calculating config memory size

In mxt_update_cfg(), the driver calculates the memory size needed to store
the configuration as data->mem_size - cfg.start_ofs. If data->mem_size is
less than or equal to cfg.start_ofs, this calculation will underflow or
result in a zero-size buffer, neither of which is valid for a configuration
update.

Add a check to return -EINVAL if data->mem_size is too small. While at it,
change the types of start_ofs and mem_size in struct mxt_cfg to u16 to
match the device address space.

Assisted-by: Gemini:gemini-3.1-pro
Link: https://patch.msgid.link/20260504185448.4055973-2-dmitry.torokhov@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>