USB: serial: garmin_gps: use struct_size() to allocate pkt

Use the struct_size macro to calculate the size of the pkt, which
includes a trailing flexible array.

Suggested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>