i2c: xiic: Remove the disabling of interrupts

Currently the interrupts are disabled at the start of the
isr and enabled at the end of the isr. Remove the same.

In case the slave device NACKs the transaction while in the isr
the transfer will continue and the NACK interrupt will arrive
only after the isr is serviced.

Signed-off-by: Shubhrajyoti Datta <shubhraj@xilinx.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>