Speed up the USB IRQ handler

[gorlik]

Avoid reading ISTR multiple times which is quite slow on L0 MCUs
Change follows the same pattern in PCD_EP_ISR_Handler. Cache the value in wIstr and do repeated checks on the variable.

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[ALABSTM]

Hi @gorlik,

Thank you for this proposal. It will be logged into our internal database and submitted to our development teams. I will keep you informed.

With regards,

[ALABSTM]

ST Internal Reference: 105754

[ALABSTM]

Thank you for this contribution @gorlik.