mfd: omap-usb-host: Increase size of buffer to include all possible values

Avoid these nasty W=1 errors:

drivers/mfd/omap-usb-host.c: In function ‘usbhs_omap_probe’:
drivers/mfd/omap-usb-host.c:706:54: error: ‘_clk’ directive output may be truncated writing 4 bytes into a region of size between 1 and 11 [-Werror=format-truncation=]
drivers/mfd/omap-usb-host.c:705:17: note: ‘snprintf’ output between 24 and 34 bytes into a destination of size 30
drivers/mfd/omap-usb-host.c:721:56: error: ‘%d’ directive output may be truncated writing between 1 and 11 bytes into a region of size 8 [-Werror=format-truncation=]
drivers/mfd/omap-usb-host.c:721:33: note: directive argument in the range [-2147483640, 2147483647]
drivers/mfd/omap-usb-host.c:720:17: note: ‘snprintf’ output between 28 and 38 bytes into a destination of size 30
drivers/mfd/omap-usb-host.c:731:55: error: ‘%d’ directive output may be truncated writing between 1 and 11 bytes into a region of size 9 [-Werror=format-truncation=]
drivers/mfd/omap-usb-host.c:731:33: note: directive argument in the range [-2147483640, 2147483647]
drivers/mfd/omap-usb-host.c:730:17: note: ‘snprintf’ output between 27 and 37 bytes into a destination of size 30

Cc: Tony Lindgren <tony@atomide.com>
Cc: Keshava Munegowda <keshava_mgowda@ti.com>
Cc: Roger Quadros <rogerq@ti.com>
Cc: linux-omap@vger.kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>