[efi] Allow calls to efi_snp_claim() and efi_snp_release() to be nested

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/efi/efi_snp.h

 extern void efi_snp_hii_uninstall ( struct efi_snp_device *snpdev );
 extern struct efi_snp_device * find_snpdev ( EFI_HANDLE handle );
 extern struct efi_snp_device * last_opened_snpdev ( void );
-extern void efi_snp_set_claimed ( int claimed );
+extern void efi_snp_add_claim ( int delta );
 
 /**
  * Claim network devices for use by iPXE
  *
  */
 static inline void efi_snp_claim ( void ) {
-	efi_snp_set_claimed ( 1 );
+	efi_snp_add_claim ( +1 );
 }
 
 /**
  *
  */
 static inline void efi_snp_release ( void ) {
-	efi_snp_set_claimed ( 0 );
+	efi_snp_add_claim ( -1 );
 }
 
 #endif /* _IPXE_EFI_SNP_H */

src/interface/efi/efi_snp.c

 }
 
 /**
- * Set SNP claimed/released state
+ * Add to SNP claimed/released count
  *
- * @v claimed		Network devices are claimed for use by iPXE
+ * @v delta		Claim count change
  */
-void efi_snp_set_claimed ( int claimed ) {
+void efi_snp_add_claim ( int delta ) {
 	struct efi_snp_device *snpdev;
 
 	/* Claim SNP devices */
-	efi_snp_claimed = claimed;
+	efi_snp_claimed += delta;
+	assert ( efi_snp_claimed >= 0 );
 
 	/* Update SNP mode state for each interface */
 	list_for_each_entry ( snpdev, &efi_snp_devices, list )