usb: phy: samsung: Fix wrong bit mask for PHYPARAM1_PCS_TXDEEMPH

According to the datasheet, PHYPARAM1_PCS_TXDEEMPH is set as
6 bits [5:0]. Thus, the bit mask should be set as 0x3f, instead
of 0x1f.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>