usb: f_serial: Check for SMD data length in GSER_IOCTL

If user tries to send SMD data more than the driver
buffer can handle then fail the same and print
error message. This smd_write is exposed to userspace
through ioctl using a misc device.

Change-Id: Ie8a1c1c0799cd10cef512ad6b1e1e95001dd43b2
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>