usb: phy: tahvo: fix smatch warnings

phy-tahvo introduced the following smatch warnings:

drivers/usb/phy/phy-tahvo.c:203 tahvo_usb_set_host() warn: variable dereferenced before check 'otg' (see line 199)
drivers/usb/phy/phy-tahvo.c:235 tahvo_usb_set_peripheral() warn: variable dereferenced before check 'otg' (see line 231)

Fix by deleting bogus NULL pointer checks. The USB framework will always
call us with a valid OTG pointer.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Felipe Balbi <balbi@ti.com>