tty: serial: msm_serial_hs: Enable/Disable uart irq over clk on/off

Keeping UART IRQ enabled all the time will gate XO shutdown in idle
power collapse. Instead, handle enable/disable of irq within clk on/off
functions. Also, making sure that wakeup irq is enabled before uart irq
is disabled.

CRs-Fixed: 606758
Change-Id: I5726fe6811e735ff3dac8d88ea86fd50fda0a61e
Signed-off-by: Girish S Ghongdemath <girishsg@codeaurora.org>