ehci-atmel: fix compiler warning

Commit 7a101e946cba55e32d3d1265e30456c810046da3 introduced following warning:
---8<---
ehci-atmel.c: In function 'ehci_hcd_init':
ehci-atmel.c:49:2: warning: suggest parentheses around comparison in operand of '&' [-Wparentheses]
ehci-atmel.c: In function 'ehci_hcd_stop':
ehci-atmel.c:79:2: warning: suggest parentheses around comparison in operand of '&' [-Wparentheses]
--->8---

This patch fixes it.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
cc: Bo Shen <voice.shen@atmel.com>
cc: Marek Vasut <marex@denx.de>