ipack: ipoctal: fix missing allocation-failure check

ipack: ipoctal: fix missing allocation-failure check

Add the missing error handling when allocating the transmit buffer to
avoid dereferencing a NULL pointer in write() should the allocation
ever fail.

Fixes: ba4dc61fe8c5 ("Staging: ipack: add support for IP-OCTAL mezzanine board")
Cc: stable@vger.kernel.org      # 3.5
Acked-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210917114622.5412-5-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>