usb: phy: isp1301-omap: fix smatch warnings

phy-isp1301-omap produces the following smatch warnings:

drivers/usb/phy/phy-isp1301-omap.c:1280 isp1301_set_host() warn: variable dereferenced before check 'otg' (see line 1278)
drivers/usb/phy/phy-isp1301-omap.c:1336 isp1301_set_peripheral() warn: variable dereferenced before check 'otg' (see line 1334)
drivers/usb/phy/phy-isp1301-omap.c:1417 isp1301_start_srp() warn: variable dereferenced before check 'otg' (see line 1414)
drivers/usb/phy/phy-isp1301-omap.c:1445 isp1301_start_hnp() warn: variable dereferenced before check 'otg' (see line 1442)

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

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Felipe Balbi <balbi@ti.com>