usb: gadget: s3c-hsotg: fix can_write limit for non-periodic endpoints

Value of can_write variable in s3c_hsotg_write_fifo function should be limited
to 512 only for non-periodic endpoints. There was some discrepancy between
comment and code, because comment suggests correct behavior, but in the code
limit was applied to periodic endpoints too. So there is additional check
causing the limitation concerns only non-periodic endpoints.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>