PCI: dwc: Improve code readability and simplify mask/unmask operations

Improve code readability and simplifies mask/unmask operations by
inverting the applied logic (no functional change is intended).

Replace variable name from irq_status to irq_mask, since its goal is to
keep track of which interrupts are masked or not.

Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: Jingoo Han <jingoohan1@gmail.com>