drivers: mailbox: qti_tcs: Do not use threaded IRQ

The mailbox driver uses spin_lock to check if the TCS is free. The TCS
will be freed only after receiving an interrupt. Currently the interrupt
handler is requested as a threaded IRQ. If the caller was from another
threaded IRQ context of higher priority or hard IRQ context, then our
interrupt handler will never get a chance to run and will forever wait
for the TCS to be free.

Change-Id: I89922e9fe48f4bd33c06fee108ebde65c5137bd2
Signed-off-by: Lina Iyer <ilina@codeaurora.org>